Electronically controlled slot antenna-amplifier



Dec. 29,1970

A- G. JENNETTI ELECTRONICALLY CONTROLLED SLOT ANTENNA-AMPLIFIER Filed Jan'. 1 6} 1967 2 Sheets-Sheet 2 E C D 0 2 2 m B m .m 2 b 1 ||l/| 11 :1} Milk 5 H J f v I wi \/\/\/\/I I 111. .Il:.|.| d 3 7 M C m D P m 2 5- FIG. 3

INVENTOR ANTHONY G. JENNETTI ATTORNEY United States Patent US. Cl. 325-374 5 Claims ABSTRACT OF THE DISCLOSURE A tunable slot antenna-amplifier, the gain of which is electronically variable over a wide range. The radiating element consists of a probe-fed, cavity-backed slot antenna, a transistorized amplifier being directly matched into the probe feed system.

This invention relates generally to antenna structures, and more particularly to integrated antenna systems.

An antenna design is said to be integrated when the antenna structure performs circuit functions as well as antenna functions. The use of the antenna as a circuit element is desirable in that the need for transmission line and associated matching elements between antenna and the receiving or transmitting circuitry, is thereby obviated. This reduction in components results in a more compact and lighter-weight design, a lowering of losses, and hence an improvement in the noise performance of the system.

In the copending application of C. H. Walter and K. Fujimoto entitled integrated Antenna-Amplifier, Ser. No. 449,910 filed Mar. 15, 196-5, abandoned in favor of continuation application Ser. No. 775,589, filed Nov. 12, 1968, now Pat. No. 3,496,566, and assigned to the same assignee as the instant application, there is disclosed a form of integrated antenna design in which an amplifier is integrally combined with a dipole antenna to form a so-called antenna-amplifier. While the the design therein disclosed has many of the advantages previously cited for integrated designs, dipoles are of but limited use in many modern applications of antenna structures, as for example in airborne environments where the dipole structures lack necessary mechanical stability and in addition interfere with aerodynamic flow. The absence in dipoles of mechanical stability and rigidity also becomes an important limiting factor in phased-array applications where rigid elements are needed to reduce variations in element spacing.

In accordance with the foregoing it may be regarded as an object of the present invention to provide an integrated antenna structure suitable for electronically controlled array applications.

It is a further object of this invention to provide a design wherein antenna and amplification functions are integrated into a single slot antenna structure.

It is an additioinal object of the present invention to provide an integrated antenna design incorporating a probe-fed slot antenna.

It is another object of the present invention to provide an integrated antenna design possessing excellent mechanical stability and rigidity.

It is yet a further object of the present invention to provide an integrated antenna structure which, because it may utilize the existing structure of an aircraft, is particularly suitable for air-borne applications.

Now in accordance with the present invention, these objects, and others as will become apparent in the course of the ensuing specification, are achieved through c0- operative use of a radiating element in the form of a probe fed cavity-backed slot antenna, a transistorized RF amice plifier designed into the probe feed system, and stripline output circuitry including a matching circuit tunable over a wide range.

A fuller understanding of the present invention may now best be gained by a reading of the following detailed specification and by a simultaneous examination of the drawings appended hereto in which:

FIG. 1 is a simplified frontal view of an antenna-amplifier designed in accord with the present invention.

FIG. 2 is a side sectional view of the FIG. 1 apparatus and schematically depicts the principal functional components thereof.

FIG. 2a is an enlarged view of the strip-line shown in FIG. 2.

FIG. 3 is a simplified analytical model of the BIG. 2 showing, and will \be found useful in analysis of the present invention.

FIG. 4 is a detailed partially sectioned showing of the stripline D.C. biasing and coupling scheme utilized in the FIGS. 1 and 2 apparatus.

FIGS. 5 and 6 show curves illustrating typical gain and phase-shift obtainable from a device designed in accord with the present invention.

FIG. 7 is a graph illustrating a typical bandwidth response obtainable for an antenna-amplifier designed in accord with the present invention.

The general physical configuration of the present inventive apparatus appears in the isometric view of FIG. 1. It is seen therein that the antenna structure is a cavitybacked slot. This type of antenna is particularly suited for airborne applications because it is capable of utilizing the existing structure of an aircraft and does not unduly disturb aerodynamic performance. In addition such antennas possess excellent mechanical stability and rigidity, a consideration which becomes important in phased array applications, particularly in the microwave region and in superdirective array applications where rigid elements are needed to reduce variations in element spacing. The cavity 5 is probe-fed in a manner that will best be apparent in connection with the ensuing description of FIG. 2. However, it will be immediately apparent that feeding the structure via a probe feed in cavity 5 is once again an ideal arrangement for airborne applications in that the feed terminals are away from the antenna aperture and so do not disturb the surface of the aircraft.

The FIG. 1 apparatus is shown in schematic side sectional view in FIG. 2. As seen therein the cavity 5 is fed by a transistor amplifier 7. The latter may typically comprise a grounded-base coaxial package such as a device available commercially from the Philco Corporation, Philadelphia, Pa., under the designation Philco L- 5431. The transistor is flush-mounted on wall 9 of cavity 5 with the emitter and collector terminals being short probes. The emitter probe is extended into cavity 5 by conductor 11 to form the input to transistor amplifier 7.

The antenna-amplifier is designed so that the impedance of the antenna at the probe feed is the complex conjugate of the transistor input impedance. Of primary importance in the design of a probe-fed slot antenna of this type are the properties of waveguide feeding junctions since the aperture admittance and waveguide parameters are well-known. In this connection reference should be made temporarily to FIG. 3 wherein a device is schematically depicted which while functionally equivalent to portions of the FIGS. 1 and 2 apparatus, should be regarded as merely constituting an analytical model. For the configuration depicted it can be shown that the input impedance, z is given by the expression:

where i and i are as defined in FIG. 3, Z is the H mode uide impedance,

and

is a modified Bessel Function of the second kind.

It will be clear from the foregoing equations that the number of variables present permit a very wide range of input impedances to be obtained. It can also be seen that it is possible to obtain a relatively high input impedance with a short probe. Also by appropriately choosing the optimum probe-short length I the antennaamplifier is capable of operation over a moderate bandwidth without returning, typically (where dimensions are as will later be exemplified) on the order of 200300 mc. at 1 gc., while still retaining the feature of compactness.

With continued reference to the schematic equivalent shown in FIG. 3, the input signal is received into the mouth of the cavity 5 as shown by the major arrow. Interposed from the lower wall of the cavity 5 is the input probe circuit including transistor 7 amplifier. Specifically, probe circuit 15 comprises outer conductor 14 and inner conductor 11. In this illustration inner conductor 11 is the emitter of the transistor 7. Slideably attached across the extreme end of the probe 15, outer conductor 4 and inner conductor 11 is the shorting bar 33.

In operation, the input circuit is a tuned amplifier circuit. In view of the frequency of the circuit lumped capacitors and inductors are not necessary since the probe comprising outer conductor 4 and inner conductor 11 has sufficient capacitance and inductance to be selfresonating. DC. bias is applied to the emitter 11 of transistor 7 from a DC. source through feed-through capacitor 23 and the A wave shorted high-impedance line, here shown in the form of a choke 27.

Capacitor 30 is merely inserted in the circuit to prevent the DC. bias from being shorted to the shorting bar 33. As will be seen with respect to FIG. 2 the shorting bar 33 is substituted for by a tuning capacitor 17.

Adjustable short 21 positioned in the cavity 5 also provides another adjustment for the input circuit.

The base 8 of the transistor 7 is shorted to the wall of the cavity at 8a and thereby is grounded. The collector 10 is in the output probe circuit 13. The output circuit 13 is substantially similar to the input circuit 15. Specifically, the circuit is tuned by the double-stub 12-16 matching circuit in a manner similar to that of input probe 15. No lumped capacitors or inductors are utilized. DC. bias is applied to the collector 10 by way of feedthrough capacitor 24 and choke 14. Capacitor is merely in the circuit to prevent DC. from being inserted in the output terminals 18.

Again, as will be seen with reference to FIG. 2, the shorting stubs 12-16 are substituted for by a tuning capacitor 19.

In both the input circuit 15 and the output circuit 13, it is seen that the A wave shorted high impedance lineschokes 27 and 14-per-mit the passage of D.C. without the loss of either the input or the output signals.

Returning now to FIG. 2 it will be seen that all circuitry external to cavity 5 is actually in the form of striplinetypically of A-inch width. Thus the transistor output circuitry consists of a double-stub stripline tuner network 13 mounted on one of the outside cavity walls of the antenna, the network contacting the collector probe of amplifier 7 at point 8. Similarly, the input tuning circuit 15 is formed from stripline. A standard coax to stripline transition terminal connects network 13 to a coaxial output line, but is not explicitly shown in FIG. 2 as it is out of the plane of the drawing but shown in FIG. 3 as connector 18. Miniature tubular capacitors of conventional design are inserted in the stripline in substitution of both the double-stub matching circuit in the output circuit and the probe input circuit to serve as the tuning elements. One of a pair of such tuning elements is shown at 17 in the network 13; a similar tuning element appears at 19 in the probe input circuit 15. In result such elements produce the same effect as varying adjustable length transmission line shorts such as variable short 33 of FIG. 3. It may be noted that Waveguide adjustable short 21 is also attached at one end of the cavity 5. Since the probe diameter is fixed, this additional short provides another variable.

As shown conventionally and commercially, a strip line is a pair of dielectric material backings with a metallic plate on its one side. On the other side is the conductor wire. The two sides are then fixed together with the inner conductors face to face.

The technique for feeding D.C. transistor bias into the stripline and also achieving A.C. coupling is illustrated in the exploded view of FIG. 4. DC bias is fed into the stripline conductor 17a through A.C. bypass D.C. feed-through button capacitors as at 23; the bias being applied to the stripline center conductor 17a through a shorted high-impedance stub 27. The one end of the feed through 23a is shorted to the plate 25 and the doughnut shaped button 23b is shorted to the plate 29. The technique for A.C. coupling illustrated in FIG. 4 consists of forming a coupling capacitor between the two stripline center conductors 17a and 17b of FIG. 2a and through the insertion of a Mylar dielectric strip 28. The gaps in the elements 17 and 17a act as the capacitors 20 and of FIG. 3, Le, to permit the unimpeded input and output signals but without the return flow of DC.

An antenna-amplifier in accordance with the foregoing principles and intended for operation in the 1 gHz. region has been designed wherein the dimensions of the slot (in FIG. 1) are 3 cm. x 24 cm. The total depth of cavity 5 is 8.45 cm. End portion 6 has dimensions 37 cm. square. Performance characteristics have been experimentally determined for the device and are illustrative of the high order of efiiciency obtainable by the invention.

Of first interest is the gain of the constructed antennaamplifier. Gain is defined as the relative increase in power at the load resulting from insertion of the transistor into the antenna. In order to measure the gain, the antenna-amplifier is compared to a properly matched slot antenna of the same dimensions. The maximum gain observable with this method was 7 db at l gHz. This gain figure agrees with the theoretical maximum available gain of the transistor based on z-parameter analysis which also is 7 db.

One application of antenna-amplifier 3 is as an element in :an electronically controlled array of slots in a ground plane. Amplitude taper in the array can be obtained through changing the DC bias voltage or cur- [rent in the antenna-amplifier, thus achieving electronic gain control. Electronic gain control may be obtained in the exemplary antenna-amplifier through either emitter current control or through collector-emitter voltage control. Gain control properties for both methods are given in FIGS. 5 and 6 for collector an emitter control, respectively. Both methods yield about db of dynamic gain variation from 7 db gain to 13 db attenuation.

It has been foundthat the relative phase shift across the amplifier will change with either gain control scheme mentioned above because of changing transistor parameters. This differential phase shift is illustrated also in FIGS. 5 and 6'. Although both methods yield a total gain variation of over 20 db (7 db amplification to 13 db attenuation), the emitter current-gain control produced a very small differential phase shift of 2% maximum deviation over the entire range.

The bandwidth of the exemplary antenna-amplifier is illustrated in FIG. 7. It is 12 me. to the half-power points. The antenna-amplifier is tunable over approximately the range from 900-1150 MHz. with no appreciable loss in gain. Tuning is accomplished through adjusting the tubular capacitors in the double-stub tuner and also the cavity depth. The adjustable stripline short fixed to the probe need not be tuned. The bandwidth limits of the exemplary antenna-amplifier, if the Waveguide short is not moved, is approximately 250 MHz. with about a 3 db maximum loss from the maximum gain figure at the band edges centered around 1 gHz.

In the foregoing description electrical and physical parameters such as impedances, power values, dimensions etc., have from time to time been assigned specific values. It will, of course, be understood by those skilled in the art that specification of parameters in this manner is merely for the purpose of furnishing positive illustration of the invention, and is not intended to delimit the invention otherwise set forth. Additionally, it will be observed that the antenna-amplifier description adheres generally to a terminology most appropriate to a receiving mode of operation. Again this facet of the specification should be understood as Ibeing only illustrative, the invention being in all respects equally suited to a radiating mode of operation.

Finally, while the present invention has been particularly described in terms of specific embodiments thereof, it will be understood that in view of the present disclosure numerous modifications thereof and variations thereupon may now be readily devised by those skilled in the art without yet departing from the teaching set forth. Accordingly the invention is to be broadly construed and limited only by the scope and spirit of the claims now appended hereto.

What is claimed is:

1. An integrated antenna-amplifier structure comprising: a cavity-backed probe-fed slot antenna and a transistorized amplifier directly matched into said pro'be feed, said amplifier comprises a single transistor flushmounted on the wall of said antenna cavity, the emitter of said transistor being extended into said cavity to form the input to said amplifier, a stripline output circuitry mounted on the exterior wall of said cavity, said circuitry being electrically connected to the output from said amplifier, said output circuitry including a matching circuit tunable over a wide range.

2. Apparatus according to claim 1 wherein the cavity of said antenna includes an adjustable Waveguide short.

3. Apparatus according to claim 1 further including a shorted quarter Wave-length high impedance stub and an A.C. bypass button capacitor electrically connected between the center conductor of said stripline output circuitry and the wall of said cavity whereby bias voltage may be applied to said amplifier by application of said voltage to said stub, whereby electronic gain control of said antenna-amplifier is enabled.

4. An integrated antenna-amplifier structure comprising: transistor amplifier means and a cavity-backed slot antenna directly coupled to said amplifier means, the impedance of said amplifier means at the coupling point being the complex conjugate of the impedance of said antenna at said point; a stripline matching network mounted on the wall defining said cavity and electrically coupling the output of said amplifier means to a coaxial transition; a tunable stripline input network mounted on said Wall and electrically coupled to the input to said amplifier means; and biasing means for said amplifier means whereby the gain of said antenna-amplifier may be electronically controlled.

5. Apparatus according to claim 4 wherein said biasing means is electrically coupled to said amplifier means through said stripline networks.

References Cited UNITED STATES PATENTS 3,394,373 7/1968 Makrancy 343769 3,386,033 5/1968 Copeland 343-- 1 3,343,089 9/1967 Murphy et al 343701 3,261,018 7/1966 Mast 343-789 2,761,139 8/1956 Dillon et a1. 343769 2,724,772. 11/1955 Bridges et a1. 343789 ELI LIEBERMAN, Primary Examiner US. Cl. X.R. 343-701, 768, 789 

